contain variou e interpreted as most V atoms on the catalytically active surface have the 4+ oxidation state, but one cannot conclud
s kinds of surface and bulk defects. Therefore, the results should b
e from the data anything about a role in the catalytic process of a minor amount of V atoms in other oxidation states. Although the estimated oxidation state of the active material matches the oxidation state of vanadyl pyrophosphate crystalline phase, this data cannot be viewed as a confirmation of the fact that the active phase is vanadyl pyrophosphate. On the contrary, one can suggest that the vanadyl pyrophosphate structure is not the structure of the active surface. Literature data for reduction of VPO in H2 and n-butane show formation of the (VO)2P2O7 bulk phase and no V3+-phase formation was reported [114, 115]. This means that the vanadium pyrophosphate phase has a high stability towards reducing conditions, while our investigation shows fast reduction of the active layer in n-butane atmosphere for both catalysts. Moreover, numerous in situ and ex situ studies revealed (VO)2P2O7 as the main bulk phase of a well-equilibrated VPO catalyst, which means that the vanadium pyrophosphate phase is stable towards changing from reaction conditions to room temperature. At the same time, in situ XAS data [79, 81, 82] clearly show that the electronic structure of the VPO surface varies dynamically with conditions. The same observations would suggest none of the well-ordered crystalline structures as the active phase. Furthermore, several experimental studies reveal amorphous nature of the active surface [79-80]. The permanence within the experimental error of a P/V ratio during our experiments allow us to suggest rather the phosphate nature of the VPO active layer, but not a two-dimensional vanadium oxide, which was proposed recently [79]. The main role of phosphor in the operation of the catalyst is believed to be the setting of the correct relative position of vanadium active centers because the reaction of n-butane oxidation to MA was shown to take place also on supported vanadium oxides without presence of phosphor [116]. One can assume two factors. Firstly, it is insulation of oxygen domains, known as site insulation [89]. Secondly, phosphor could set a distance between two neighbor vanadium atoms so, that it fits the size of the n-butane molecule and consequently, the molecule can be easier accommodated [93].
As it was shown in this work, the composition of the surface layer can be different from the bulk that without a significant difference in the catalytic performance compared to the performance of a homogeneous VPO material. This fact clearly points to the development of supported vanadium oxides as a way of further improvement of a catalyst for the steady-state catalytic process. Several groups are involved in investigation of supported VPO [61, 62, 63]. Although a catalytic performance of produced samples is still far away from a bulk commercial catalyst, this field of activity looks very promising first of all, because of the
principal possibility to employ a cheap support material instead of complicated prepared bulk VPO and because of expected significant decrease of the catalyst activation period.
Additionally, study of alumina supported vanadium oxides [116] shows a principal possibility to produce maleic anhydride from n-butane without use of phosphates, which would be another option for developing an improved catalyst for this reaction.
Another type of VPO catalyst is the catalyst for the unsteady-state DuPont process, i.e.
the recirculating-solids process. A catalyst employed in this process should satisfy additional requirements comparing a catalyst for the steady-state process [117]. The material should readily supply bulk oxygen to the surface to make possible long-time catalyst operation under reducing conditions. Moreover, the material should be easily reoxidizable in the regeneration part of the operating cycle, which also implies good oxygen exchange between the gas phase, the surface and the bulk. In situ XPS is the unique technique, which provides a possibility to perform direct studies of lattice oxygen exchange in a near-surface region. Such studies were performed on bulk vanadium oxides [118] and can be readily performed on supported vanadium oxides [116]. The studies in the present work were performed in the reducing n-butane/He atmosphere. Change with time of a vanadium oxidation state of the samples correlates well with change of the MA yield. The sample having a better homogeneity with depth showed slower changes in oxidation state and in MA yield compared with the other sample. This implies better interaction of the active surface layer with the bulk in the sense of oxygen exchange, which immediately leads us to the conclusion about better suitability of a material of this catalyst towards condition changes in the unsteady-state process. These results show the power of in situ XPS method for the investigation of interaction phenomena of the active layer and a substrate, which would be extremely important for designing of supported catalysts for unsteady-state catalytic processes.
maleic anhydride production from
e of the main factors influencing the catalytic performance of the catalyst was not found to have any systematic changes during change of conditions. Scattering of the data points was within the experimental error. An absolute value of P/V ratio was calculated using experimental correction coefficients to theoretical sensitivity factors. The coefficients were estimated using XPS data of some solid reference compounds.
An average P/V ratio of the VPO sample was found to be close to unity and did not change with depth. The uncertainty in the sensitivity factors does not allow formation of strong conclusions about the phosphor enrichment or depletion on the surface. More precise measurements can be done using gas-phase reference compounds, as was discussed in part 4.8.
The process of supplying the active layer with oxygen from the bulk under reducing
5 Conclusions and outlook
An experimental setup for in situ XPS investigations of the catalyst's surface was constructed. Physical principles of the system design were described in detail in part 3. In situ XPS technique was applied to a VPO catalyst, which is an important industrial catalyst for n-butane. The results of the investigation were published in [119].
Two VPO catalysts with similar intrinsic catalytic activities towards maleic anhydride were investigated. In the reaction mixture one catalyst showed a high homogeneity of vanadium oxidation state with depth and another catalyst had a significant gradient of vanadium oxidation state. At the reaction conditions (2 mbar, reaction mixture, 400°C) both catalysts had the same surface oxidation state of (4.0±0.1), while the bulk oxidation state differed significantly. The experimental results suggest that the catalytically active species of a VPO catalyst are located in the topmost layer of thickness less than (3.5 ± 2.0) nm. The structure of this layer does not necessarily match the bulk structure and the bulk material is acting as a substrate only. The finding indicates that a bulk vanadium oxidation state in general, should not correlate with catalytic properties. The concept of an active layer with structure different from that of the bulk can play the central role in the strategy of improvement of a VPO catalyst. While the modern preparation methods are designed to get the (VO)2P2O7 bulk phase, optimization of formation of the active surface layer could be more important. In this respect, development of a supported vanadium oxide catalyst looks very promising as a way to improve the catalyst and reduce its prices.
A P/V ratio, which is often referred as on
gas phase oxygen decreasing MA
yield. The sample state with depth
showed slo
of catalys
Nowadays one feels that there is a lack of such data in literature, but it is believed to be . Changes in the V2p3/2 XPS peak correlate well with the
which had a better homogeneity of vanadium oxidation
wer changes in oxidation state and MA yield compared with the other sample. This means better supply of the catalytically active layer with bulk oxygen for this sample. This finding can be important for the catalyst design for the DuPont process, where a VPO catalyst operates under reducing conditions. The results show also the power of in situ XPS method for investigation of the phenomena of lattice oxygen exchange, which is important for design
ts for unsteady-state catalytic processes.
An accuracy of the XPS method could be significantly improved by obtaining additional theoretical knowledge about the photoelectron spectral-line shapes for material with various chemical composition and structure and about the photoionisation cross-sections.
possible to obtain it in the near future.
The results of this work together with other publications ([33], [34]) demonstrates suitability and importance of the in situ XPS technique for characterization of the active surface of a solid-state catalyst including a real industrial catalyst.
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